This invention relates to vehicular braking systems having a power assist feature therefor and more particularly to an electrohydraulic brake booster system. The system may have a common boost and input chamber or a separate input chamber within the boost piston.
Early power assisted braking systems were vacuum actuated utilizing the engine intake manifold as a source of power. More recently, hydraulic assist systems have become popular. The hydraulic systems usually either rely on a power steering pump as a source of pressurized fluid or include a separate dedicated fluid pump. The hydraulic systems typically include a power assist unit or booster having a driver input piston disposed within an input cylinder and a power output piston disposed in a separate output cylinder for powering a master brake cylinder. Typically, the pressure on the working face of the input piston is monitored and the pressure applied to the working face of the output piston set accordingly. The distances traveled by the input and output pistons are largely ignored.
Brake booster systems are sometimes commanded to apply the maximum available fluid pressure for braking. Any additional brake pedal pressure fails to raise the force applied to the master cylinder. If no precautions are taken, the driver may continue to depress the brake pedal, but feel no increase in resistance to pedal motion. Finally, vehicle braking should still be possible despite catastrophic failure of the boost fluid pressure as by engine stoppage, or a belt slipping or breaking. Desirably, the system reverts to a conventional (no boost) braking mode.
It is desirable to take into account the travel of the input piston in setting output piston face pressure. Moreover, also monitoring the output piston travel allows the output pressure to be in part determined by that travel allowing variations in the ratio of input to output piston travel to be incorporated in the braking system.
The present invention provides solutions to the above concerns by providing a boost pressure system which may include linear measures of both driver input and boost output travel, may utilize a single boost chamber to supply both force to a vehicle master cylinder and opposition force to the driver input, or may employ a separate driver input force opposition from a pressure chamber enclosed within a boost piston.
The invention comprises, in one form thereof, a vehicle brake booster having a source of pressure fluid, a booster housing with first and second generally cylindrical bores therein, a driver actuable brake input piston reciprocally disposed in the first cylindrical bore, and a hydraulically powered brake master cylinder actuating output piston reciprocally disposed in the second cylindrical bore. The second cylindrical bore together with the output piston define a boost chamber with the input piston extending from the second bore into the boost chamber. A pressure fluid conduit couples the source of pressure fluid to the boost chamber, and the boost chamber is completely defined by the second cylindrical bore, the input piston, the output piston and the fluid conduit.
In another form, the invention includes a vehicle brake booster having a housing with a generally cylindrical bore and a hydraulically powered brake master cylinder actuating boost piston reciprocally disposed in the cylindrical bore. The cylindrical bore and a working face of the boost piston define a boost chamber. There is a generally cylindrical bore in the boost piston extending from the piston working face part way through the piston. A driver actuable brake input piston passes through the boost chamber and is reciprocally disposed within the boost piston bore. The boost piston bore and a working face of the input piston define an input chamber. There is a source of pressure fluid and an arrangement for selectively supplying fluid pressure from the source to the boost chamber and to the input chamber.
The invention also comprises a method of amplifying a hydraulic brake force applied by a vehicle operator in which the driver input brake force is sensed by monitoring linear motion of a driver actuable input piston. The amplified hydraulic braking force is sensed by monitoring linear motion of an output piston or pressure. Fluid from a fluid pressure source is conveyed to a boost chamber to move the output piston a distance proportional to the distance moved by the input piston to actuate a vehicle brake master cylinder in proportion to the sensed travel associated with the driver applied input brake force. The ratio of input piston travel to output piston travel may be modified.
An advantage of the present invention is that the ratio of input piston travel to that of output piston travel may be selectively modified as desired to facilitate driver pedal feel.
Another advantage of the present invention is that the system is fail-safe reverting to a completely manual mode in the event of hydraulic or electrical failure of the boost system.